Don't even consider this lens on a full frame EOS body.
3 August 2012
Summary: This very old design L glass zoom was a total disappointment. The image in the corners using a full frame Canon 5D is just awful wide open and only gets acceptable at f/16. At f/16 and f/22 diffraction softens the central image, so it is a bad trade-off. I returned this lens and later purchased a Zeiss 18mm f/3.5 Distagon T* ZE lens. Though the Zeiss is a manual focus prime lens, everything else works on any EOS body, including green "in focus" light and beep.
Little-known Gem of Canon's L-Series lenses. Amazing value.
26 February 2012
Summary: This lens is one of the classics that not many new photographers are aware of. Introduced in 1989, it originally had an MSRP of over $2300, but can now be found for a less than a quarter of that price. "L" build quality is evident, and the lens feels well balanced on crop sensor and full frame cameras alike. Color fidelity is good, and auto focus is quick and consistant.
Summary: I shoot weddings and have been using the Sigma 24 1.8 for 2 years, mainly on a cropped sensor body for its macro capability, low light capacity and descent focal range on a crop camera during reception stuff. On tight timing situations, the Sigma's AF goosed too many shots, its slow and tracks, and having experienced the USM of Canon's 50 1.4 and 100 f2, this lens was my logical choice.
Summary: This lens starts off pretty soft wide open, and has very strong vignetting even on asp-c sensors. But, once you stop down to f/4 or smaller it becomes very sharp and all the vignetting goes away. Considering this is the widest full frame lens for under $1200 it's a pretty good deal.
Summary: I have to admit that I am surprised at the sharpness and contrast of this lens. I don't know what kind of settings the two previous reviewers used but I can assure you this lens produces excellent results on Canon 30D, I own 28-70mm f/2.8, 70-200mm f/4 IS and the sharpness is about the same, as for the colors, well I am really amazed.
Conclusion: I got mine in near mint condition on eBay. It's a great lens, a true L. I also have the 24-105L which is fine, too, but this is a totally different tool. The colors are crisp and it is tac-sharp even at 2.8. on my 5DII, I normally shoot primes and use zooms only when I'm lazy or leave the bag at home. This little gem not only feels like a prime, it also shoots like one.
Pros: Color rendition, contrast, sharpness, bokeh. Lightweight but sturdy. Price I paid ( 420 Euros) is still low for what you get.
Cons: None. Zoom range is less than 2x, but the difference btw. 20 and 35mm is huge anyway.